The need to replace iron based metals to reduce the weight of automotive vehicles has led to the use of light weight metals such as aluminum and magnesium alloys. Pure aluminum can not be used due to its low melting point and strength, but by including a desirable amount of silicon with the aluminum, a suitable alloy can be prepared. Aluminum-silicon eutectics are quite common in the fabrication of engine components such as blocks and cylinder heads.
Since the 1960's, it has been known that the mechanical properties of light alloys can be greatly enhanced by reinforcing them with ceramics in the form, e.g., of fibers, whiskers, or particulate. These materials, called metal-matrix composites [MMC], are a promising family of next generation structural materials and will be playing a role in replacing metals in the fabrication of automotive components. In addition to reduced weight, the MMC based components show improved NVH, controlled thermal expansion, and improved thermal and mechanical durability.
The common methods for the fabrication of MMCs include melt stirring and pressureless liquid metal infiltration, pressure infiltration, and powder metallurgy. The selection of the optimal method to prepare MMCs depends on a number of factors including economics and the nature of the raw materials. Components of complex shape may be fabricated by casting, forging, or extrusion. For example, fabrication using the powder metallurgy method involves mixing powdered metals with reinforcing ceramics and also binders to form the green bodies which are then subjected to elevated temperatures to remove the organic binder. Each green body is then fired to obtain the component in finished form. The selection of a reinforcing material is based on economic factors, chemical stability, and desired properties.
For the automotive industry, the MMCs of present interest are based on aluminum with reinforcing particles of SiC, TiC or Al.sub.2 O.sub.3 as primary reinforcement materials in volume fractions ranging from 5 to 30 percent. The automotive industry has shown a considerable interest towards using these MMCs for fabricating a wide variety of parts including drive shafts, cylinder liners, rocker arms, connecting rods, and suspension components.
One of the drawbacks of making the MMCs is the high cost associated with the ceramic reinforcement materials. It would be desirable to provide other reinforcement materials for MMCs which are more cost effective and also provide excellent physical properties to the composites. It is important that the reinforcing particulates have good comparability with the metal, that is, that the reinforcing particulate have physical properties such as density, modulus, and coefficient of thermal expansion which are compatible with the metal to provide a strong and durable MMC. The invention disclosed herein provides MMCs with excellent physical properties and advantageously uses relatively low cost ceramic materials as the reinforcing materials.